An important application of gas sensors lies in the domain of indoor air pollution monitoring, only for obtaining objective data, and control of air handling units and/or air cleaning units. It would be desirable that such a sensor has the following features:
small size;
low cost;
low power requirements;
minimal maintenance requirements over an operational period of at least several years;
a combination of high sensitivity and high selectivity with respect to a particular target gas or target class of gases.
Selectivity is especially important in situations where the composition of the gas to be measured is not known in advance. Lack of sufficient selectivity remains a key issue with all major sensor technologies and poses severe application problems in ordinary indoor environments where usually an unknown mixture of different gaseous pollutants is present.
A high selectivity towards a specific target gas or towards a specific class of target gases allows the obtained sensor signals to be interpreted unambiguously. This aspect also applies to the influence of the air humidity, the air temperature and the local air speed on the obtained sensor signals.
As regards the quality of ambient air, it is important to be able to unambiguously distinguish clean air from polluted air. Air may be polluted by a certain gas (for instance formaldehyde, NOx, O3, SO2) or a certain class of gases (for instance the class of all volatile organic hydrocarbon gases, usually referred to as TVOC; or the class of acid gases, which includes HNOx, SO2 and organic carboxylic gases). Apart from the ability to detect whether a certain pollutant is present, it is also important to be able to detect the concentration of that pollutant. Air is considered to be unacceptably polluted with a certain target gas when the concentration of that target gas is comparable to or higher than its recommended maximum concentration limit. For indoor living environments, these so-called concentration limit standards are quite low, i.e. around 50 ppb for both O3 and NO2, 0.2-0.3 mg/m3 for TVOC and 40 ppb for formaldehyde.
At present, no sensors or sensor technologies exist that fulfill all the above requirements to a satisfactory extent. Nevertheless, the use of metal-oxide semiconducting sensors or electrochemical sensors appears to be the most promising choice in this regard. This applies in particular to the sensing of formaldehyde, which is a recognized important air pollutant, in particular in Chinese residential environments.
However, an important problem with metal-oxide semiconducting sensors and electrochemical sensors is their lack of selectivity. Several attempts to overcome this problem have already been proposed.
The gas to be examined, for instance ambient air, may contain several pollutants, and it would be desirable to be able to measure the concentration of each one of these pollutants individually. However, pollutants tend to influence measurements directed at other pollutants. In a basic approach, it is attempted to eliminate all “other” pollutants, so that only one pollutant (i.e. the target gas) remains: a sensor output signal obtained from the thus filtered gas will be proportional to the amount of (concentration of) target gas. Such an approach to try to improve the sensing selectivity of a gas sensor is described in for instance CN101825604 and CN101776640. These documents propose to specifically remove the interfering gases from air with a “scrubbing filter”.
A disadvantage of this approach is that it requires knowledge of the identity of the “other” pollutants. However, it is usually not a priori known which gaseous pollutants interfere and the extent to which they interfere with the measurement of the target gas. Furthermore, gases of widely different physical properties such as H2 and ethanol are known to be interfering gases for electrochemical formaldehyde sensors and it is far from easy to effectively remove all these gases from air at room temperature using small low-cost passive filters. It is therefore in general very difficult or even impossible to design a practical filter capable of removing all interfering gases from air while leaving everything else the same. Another approach, therefore, is to have a filter for removing the target gas from the polluted air, and to perform two measurements: one measurement on the original polluted air, which still comprises the target gas, and one measurement on the original polluted air from which the target gas has been removed. The difference between the two measurement signals obtained in these two measurements will be proportional to the amount of (concentration of) the target gas.
A company by the name of “Environmental Sensors” has recently proposed a portable electrochemical formaldehyde sensor equipped with a removable formaldehyde sheet filter impregnated with a chemical reactant capable of specifically removing formaldehyde from the ambient air entering the sensor interior (see http://www.environmentalsensors.com/formaldehyde-monitor-z-300.html). The formaldehyde filter furthermore serves as a diffusion barrier which limits the entry of gaseous species into the electrochemical cell. This formaldehyde filter can be manually replaced by a blank filter, which only serves as a diffusion barrier and which hence does not absorb any gases from air. By comparing the obtained sensor signal in the presence of the formaldehyde filter with the sensor signal in the presence of the blank filter, a signal difference is obtained that is directly proportional to the formaldehyde concentration, since the influence of other (interfering) gaseous pollutants is excluded.
A disadvantage of this approach is that the two filters can only be exchanged manually, which is inconvenient. Furthermore, the used filter is embodied as a flat fibrous sheet filter, which can be impregnated with only a very limited amount of the reactant that removes formaldehyde from air. The useful lifetime of the formaldehyde filter is therefore only short and not practical in ordinary indoor environments. It is furthermore unknown when the used formaldehyde sheet filter should be replaced. In addition, the impregnation of the fibrous sheet filter with the reactant material results in an inevitable reduction of the filter porosity, thereby changing its diffusion barrier characteristics. The latter characteristics are furthermore dependent on the ambient humidity because of the humidity-dependent moisture uptake by the reactant. The afore-mentioned circumstances result in serious interpretation difficulties with respect to the obtained signal difference in terms of the ambient formaldehyde concentration and lead to large inaccuracies.
Yet another approach to try to improve the sensing selectivity of a gas sensor is described in for instance CN101571506 (Huarui Scientific Instrument Shanghai). This document proposes an electrochemical formaldehyde sensor comprising a first working electrode, a compensation electrode, and a common counter electrode. The compensation electrode effectively acts as a second working electrode characterized in that it is provided with a filter capable of specifically removing formaldehyde from air. The formaldehyde filter furthermore acts as a general gas diffusion barrier. The first working electrode is provided with a dummy filter and only acts as a gas diffusion barrier. By subtracting the sensor signal obtained from the first working electrode (having contributions from both formaldehyde and interfering gases) from the signal obtained from the compensation electrode (having contributions from only the interfering gases), a differential signal is obtained that only accounts for the formaldehyde concentration in air and compensates for possible effects related to humidity and temperature changes.
A disadvantage of the solution offered by Huarui is that effectively two separate working electrodes are needed within a single electrochemical sensor, as illustrated in FIG. 1. Small physical differences between the two working electrodes can easily lead to quite different sensor responses and different signal bias, both with respect to their zero readings (in clean air) and with respect to their span (the signal difference per unit concentration of the target gas and/or of the interfering gases). It is therefore generally difficult, if not impossible, to unambiguously interpret the obtained differential sensor signal in terms of the target gas concentration. Because the filters are integrated within the electrochemical sensor, it is not feasible to remove or otherwise manipulate them, for instance for sensor calibration purposes.